This invention relates generally to ultrasound diagnostic imaging systems, and more particularly, to an apparatus and method for harmonic imaging using such systems.
Ultrasonic diagnostic imaging systems are in widespread use for performing ultrasonic imaging and measurements. Diagnostic images are obtained from these systems by placing a transducer assembly against the skin of a patient, and actuating one or more piezoelectric elements located within the transducer assembly to transmit ultrasonic energy through the skin and into the body of the patient. In response, ultrasonic reflections are returned from the interior structure of the body, which are converted into electrical signals by the piezoelectric elements in the transducer assembly.
Since the fluids and tissues comprising the body of the patient have a significant non-linear acoustic response when exposed to ultrasound energy, harmonic reflections are often generated within the body at one or more frequencies that are harmonically related to a fundamental transmit frequency. Harmonic imaging systems have thus been developed that emit ultrasonic energy at a selected frequency, and receive reflected or transmitted ultrasonic energy at one or more harmonic frequencies of the selected transmission frequency.
Currently, conventional broad-band transducer assemblies are used in harmonic imaging systems that are generally configured to be operable within a predetermined bandwidth that includes a range of frequencies centered about a fundamental transmit frequency. As a consequence, the transducer assembly generally exhibits favorable sensitivity at frequencies that are close to the fundamental frequency, but exhibits generally less sensitivity to frequencies near the edges of a prescribed bandwidth. Since harmonic reflections of interest often occur at frequencies near the edge of the transducer bandwidth, the sensitivity of the transducer assembly to these frequencies is often substantially reduced.
Prior attempts to achieve wide bandwidth transducer assemblies for use in harmonic imaging systems have followed at least two general approaches. One approach is to optimize the design of passive components used in the assembly, including multiple matching layers and/or backing layers to achieve broader frequency response. Transducer assemblies following this approach generally have the same frequency response when transmitting and receiving, with the ultrasound system being used to select a desired frequency response by altering the transmit waveform and/or altering the receive filter response. Since the number of passive components which can be assembled is usually very limited, this approach generally achieves only limited bandwidth improvement without compromising other performance parameters such as sensitivity.
A second approach is to optimize the design of the active components of the transducer assembly, which are generally comprised of a piezoelectric material. In one method, the piezoelectric layer material is formed with a varying thickness in a selected direction, so that the frequency response of the transducer element is broadened, as described in various publications (e.g., xe2x80x9cDual Frequency Piezoelectric Transducer for Medical Applications,xe2x80x9d M. S. S. Bolorforosh, SPIE Vol. 1733, (1992) at pp. 131 et seq.) and patents (e.g., U.S. Pat. No. 5,415,175 to Hanafy, et al.). In another method, transducer elements are fabricated with multiple layers of active transducer materials, and use a switching circuit to control the polarity of each layer or to control the signal applied to each layer, in order to generate different frequency response characteristics for the transducer elements during the transmit and receive modes. For example, U.S. Pat. No. 5,410,205 to Gururaja discloses a transducer stack consisting of two or more electrostrictive layers that may be selectively biased by applying a voltage to each layer so that the transducer transmits at one resonance frequency and receives at another resonance frequency. Further, U.S. Pat. No. 5,825,117 to Ossmann, et al. and U.S. Pat. No. 5,957,851 to Hossack also disclose transducer stacks consisting of two piezoelectric layers. Switching circuits are attached to each transducer element so that different frequency responses can be generated during the transmit and receive modes. A drawback of this approach is the requirement of additional electronic circuits associated with each transducer element to control the different modes, thus adding to the complexity of the transducer assembly.
It is therefore desirable to provide an ultrasound diagnostic imaging system that is optimized to permit the transmission of ultrasound energy at a fundamental frequency that also permits the detection of reflected ultrasound energy at harmonic frequencies with greater sensitivity than is currently obtainable from systems using conventional broad-band transducer assemblies. It is further desirable to provide a system that provides the foregoing advantages without the use of multiple transducer layers or switching elements to control the spectral response of the transducer assembly.
The invention is generally directed towards ultrasound diagnostic imaging systems, and more particularly, to an apparatus and method for harmonic imaging using such systems. In one aspect, the invention includes a transducer assembly including at least one layer formed from a piezoelectric material extending in a first direction and in a second direction that is perpendicular to the first direction and having electrodes positioned on opposing sides of the layer, the piezoelectric material being poled in the second direction, and an ultrasound processor operatively coupled to the electrodes, the processor transmitting first signals to the transducer assembly for generating ultrasonic waves, and receiving second signals from the transducer assembly corresponding to reflected portions of the ultrasonic waves, the second signals being harmonically related to the first signals. In another aspect, the invention includes a transducer assembly configured for operation in a k31 mode, a transmitter for transmitting first signals to the transducer assembly, the first signals including a first frequency, and a receiver for processing second signals received by the transducer assembly, the second signals including a second frequency harmonically related to the first frequency. In still another aspect, the invention includes a method of operating an ultrasound imaging system that includes emitting first ultrasound signals from a transducer assembly operating in the k31 mode, the first signals being confined to a first frequency range, projecting the first ultrasound signals into a body, and detecting second ultrasound signals corresponding to reflected portions of the first signals, the second signals being confined to a second frequency range different from the first frequency range.